Ok, I understand. Is it a feature to be able to join a v4 multicast group
  without having a v4 address? Are there cases where this is useful? Like
  for a DHCP like protocol?

I would say that it is a bug from a security point of view, since
there is an expectation that an interface without a v4 address will
not be able to send or receive v4 packets ("ifconfig foo0 up; tcpdump
-i foo0" is thought by some not to open one up to network attacks from
foo0 other than tcpdump overflows).  But if the addr has come and gone
this is less of an issue.

dhclient configures 0.0.0.0 for use with the socket method, and then
can either use sockets or bpf (at least earlier code could use either,
and the 0.0.0.0 still happens.

  | One could argue that having a v4 multicast group joined on an
  | interface with no v4 addrs should be illegal, and I'd be sympathetic
  | to that.  But one would have to delete all memberships from an
  | interface when the last address is removed.

  Well, if there are cases where having a multicast group joined without
  having a v4 address make sense, it probably does not make sense to
  remove the memberships when the last address is removed, but only on
  interface deletion.

Sure.  My real concern is that interface deletion is not safe.  Given
that these invariants are becoming kind of hairy, I think the only
safe thing to do (from a software maintenance viewpoint) is to have
interface destruction always scan all data structures that might have
ifnbet pointers.